1. Field of Invention
The present invention relates to a voltage control oscillator and manufacturing method thereof. More particularly, the present invention relates to a voltage control oscillator which utilizes adjustable capacitors and manufacturing method thereof.
2. Description of Related Art
In the field of electronic circuitry, work frequency of radio frequency (R.F.) is widely used in various circuits. In digital circuits, work frequency is used to coordinate different elements for achieving a predetermined task together. In analog circuits, work frequency is used to modulate or demodulate different signals for performing specific functions. Therefore, elements and circuits for generating work frequency are particularly important in circuit design.
There are various types of elements and circuits generate work frequency. One common type of such elements is a voltage control oscillator (VCO), which utilizes an external input voltage to determine and generate an output work frequency. A detailed description of a VCO including various circuit design methods can be found in the book “CMOS”, published by IEEE, 1998.
Recently, a new type of passive element, a laser trimmable capacitor, has been designed and manufactured. As shown in FIG. 1, a trimmable capacitor 10 has a trimmable area 101, and the capacitance of the laser trimmable capacitor 10 is adjusted when a laser beam is incident on several portions 1011 of the trimmable area 101. With the accuracy characteristic of lasers, the size and position of these portions 1011 can be accurately produced and the capacitance of the laser trimmable capacitor therefore can be adjusted accurately.
This kind of laser trimmable capacitors is now used in VCOs. These trimmable capacitors are connected to other circuit components of a VCO to generate a work frequency. The specification of the work frequency is therefore adjustable by burning these trimmable capacitors with a laser beam during VCO manufacturing so that a desired specification of work frequency is obtained.
However, metal covers for covering the laser trimmable capacitors and other components need to be soldered after the aforementioned frequency adjustment is done. Even if the laser beam is so accurate that the frequency adjustment is also accurate, metal covers and solders thereon randomly affect the work frequency. It is difficult to estimate the effect of the metal covers and solders thereon because the soldering process is not accurate. The problem becomes more serious when a high work frequency is needed.
More and more products in wireless communication and high speed calculation need a high work frequency. Therefore, it is very important to manufacture a voltage control oscillator with an accurate work frequency adjustment.